Colored phosphate coatings and method of application

ABSTRACT

PHOSPHATE COMPOSITIONS CONTAINING AT LEAST ONE COPPER SALT ARE UTLILIZED FOR COATING SURFACES OF FERROUS METALS. THE PHOSPHATE COATINGS ACHIEVED, WHICH POSSESS A REDDISH COLOR, SERVER AS AN EXCELLENT MEANS OF IDENTIFICATION OF THE ARTICLES THIS COATED AND, IN ADDITION, THE FINISH IS OF GOOD CORROSION RESISTANCE.

United States Patent ABSTRACT OF THE DISCLOSURE Phosphate compositions containing at least one copper salt are utilized for coating surfaces of ferrous metals. The phosphate coatings achieved, which possess a reddish color, serve as an excellent means of identification of the articles thus coated and, in addition, the finish is of good corrosion resistance.

This invention relates to aqueous phosphate coating compositions and to a method for producing phosphate coatings on metals. More particularly, it relates to zinc and zinc-iron phosphate compositions containing at least one copper salt and to a method for producing a reddishcolored zinc phosphate coating on the surface of a ferrous metal. The colored coating resulting when the aqueous solutions of this invention are employed to coat metals serves as an excellent means of identifying such articles and such coatings exhibit a high degree of corrosion resistance.

The identification of certain articles by using color coding techniques has long been implemented in the metal finishing industry. After providing corrosion resistant coatings to metals, it is often necessary and desirable to impart a color to the coated article.

' The art is aware of many systems which can be utilized to produce colored final finishes on metal surfaces including: zinc and cadmium plating followed by chromating and dyeing, aluminum chromating and dyeing, aluminum anodizing and dyeing. The above systems serve two purposes: (1) they are corrosion resistant and (2) they are colored for identification and/or appearance as a final finish. The above-mentioned chromate conversion coatings and anodized aluminum finishes are such that they lend themselves to dyeing by the use of conventional dyes and thus color coding of these articles presents no problems.

When a zinc or zinc-iron phosphate is employed as a corrosion resistant coating on metals, such coatings cannot be dyed in the same manner as the above-described chromated and anodized coatings. Methods for coloring phosphate coatings described in the art involve the use of a subsequent conversion coating of the phosphate to a state in which it can be dyed. Other methods employed for coloring phosphate coatings involve the use of a thin film forming resin pigmented with the desired color.

Ferrous metal parts which most often require a distinguishing color or color coding are threaded fasteners. In the past, the usual practice has been to employ a pigmented resin over the conventional zinc phosphate coating. Although this system has proven itself to be a somewhat satisfactory means of color coding fasteners, because of the accumulated film in the threaded areas resulting from the additional coating applied it is necessary to take special handling precautions when processing metal pieces in this fashion. The accumulation or build up of the film in the threaded areas causes deviations in the fastener tension at given torque loads. Further, in processing fasteners utilizing the pigmented resin systems, it is also necessary, to completely dry the pieces prior to placing them in 3,647,568 Patented Mar. 19 72 'ice a water soluble rust preventing oil which is usually the final step in such a phosphate coating process. Thorough drying of bulk processed work is not easily accomplished where large loads are involved.

One of the objects of thisvinvention is to produce a colored zinc phosphate coating on the surface of a ferrous metal directly from a coating bath itself without the employment of subsequent mordant and dyeing operations or the use of pigmented resin coatings.

Another object of this invention is to form phosphate coatings on the surfaces of ferrous metals which are highly resistant to normal corrosion.

In the process of this invention ferrous metal surfaces are simultaneously provided with corrosion resistant and color coded phophate coatings by treating them first in a bath comprising an aqueous zinc phosphate solution or bath containing at least one soluble copper salt. Thereafter, the initially coated and colored metal surfaces are subjected to a second phosphating treatment by immersing them in a zinc phosphate solution of generally standard commercial composition, free of dissolved copper.

The initial copper-containing zinc phosphate solutions of this invention soluble copper salts providing the equivalent of from about 0.15 to about 2.5 g./l. of copper. These initial solutions are prepared by adding at least one soluble copper salt to any of the various aqueous zinc phosphate solutions known in the art to be useful in forming protective coatings on ferrous metal surfaces. Typically the zinc phosphate solutions of the art contain zinc, iron and phosphate ions. Additionally, these solutions may contain an accelerator or oxidizing agent, such as nitrate ions, nitrite ions, chlorate ions, peroxide ions, etc. Other modifying ions may also be present such as nickel ions, manganese ions, alkaline earth metal ions, such as calcium ions, etc.

Preferably, the aqueous initial copper-containing zinc phosphate solutions utilized in this invention have the following characteristics:

Free acidity: about 3.5 to about 25 points 1 Total acidity: about 14.5 to about points Ferrous iron (Fe++): about 0.1 to about 8 g./l. Zinc (Zn++): about 1 to about 25 g./l. Phosphate (PO about 5 to about 55 g./l. Nitrate (NO about 2 to about 50 g./l. Copper (Cu++): about 0.15 to about 2.5 g./l.

The points of total acid denote the number of mls. of 0.1 N sodium hydroxide required to neutralize a 10 ml. sample of the bath to a phenolphthaleln indicator end point.

In preparing the copper-containing phosphate solution of this invention generally the ferrous iron is supplied as ferrous phosphate, or ferrous sulfate, zinc as zinc nitrate, zinc phosphate or zinc chloride, phosphate as phosphoric acid or zinc phosphate and nitrate as zinc nitrate. The selection of a particular copper salt for the phosphate bath of this invention is not critical and any copper salt which is soluble in the phosphate bath, such as cupric sulfate, cupric nitrate, cupric chloride, etc., may be employed provided the associated anion is functional (e.g., nitrate) or non-detrimental to the coating process.

In the process of this invention, a workpiece, for example a steel panel, is first cleaned by vapor degreasing, solvent cleaning, sand blasting or, preferably, by immersion in an alkaline cleaner, to remove grease, oil and dirt from the surface. Then the panel is rinsed in cold water, optionally pickled in an acid, such as sulfuric acid, phosphoric acid or hydrochloric acid of about 10 percent by volume, at about F., again rinsed in water and placed in the initial copper-containing aqueous phosphate coating bath described above for about 0.5 to about 60 minutesor more and preferably for about 10 to about 25 minutes. The temperature of the phosphating bath can be varied over a wide range and generally will be from 120 to about 210 F. and, preferably, will be about 150 to about 200 F. depending upon the particular bath employed and the other reaction conditions. The coated panel is then removed from the coating bath and rinsed with water. The result is a good quality, tight-grained, red-colored, zinc-iron phosphate coating. This color of the phosphating coating thus-formed is easily distinguished from the standard gray-colored zinc-iron phosphate coating of the prior art.

This preliminarily coated and colored panel is then immersed in, or otherwise contacted with, a typical zinc phosphating solution for a period of about 10 to about 60 minutes or more at a temperature ranging from about 170 to about 210 F.

Typical zinc phosphate solutions referred to above for use in this second step have the following characteristics:

Free acidity: about 3.5 to about 25 points Total acidity: about 14.5 to about 100 points Ferrous iron (Feabout 0.1 to about 8 g./l. Zinc (Zn++): about 1 to about 25 g./l.

Nitrate Now; about 1 to about 50 g./l. Phosphate (PO about 5 to about 55 g./l.

The result is red-colored, zinc phosphate coating.

The foregoing two-step process can be conveniently handled in existing process lines by incorporating a single station processing tank of small size to impart the red color to the steel pieces prior to immersion in the usual zinc phosphate coating bath. This two-stage processing technique yields red-colored zinc phosphate coated pieces which, after being sealed by dipping in a chromate solution and after being immersed in a water-soluble, rust preventive oil, pass all the necessary torque-tension requirements and the neutral salt spray requirements when tested according to the procedure of ASTM B117, a standard test method accepted by the industry.

A suitable chromic acid solution for sealing the coated article contains about 1 oz. to 16 oz. of chromic acid per 100 gals. of water, and the solution is preferably maintained at a temperature of about 70 to 180 F. Application of the rust preventing oil can be accomplished in known manner by immersing the pieces in a water soluble or solvent cut-back corrosion resisting oil at a temperature of about 70 to 200 F. for a period of about /2 to 5 minutes or more.

The following examples illustrate various embodiments of this invention and are to be considered not limitative:

EXAMPLE I A copper-containing phosphating solution having the following characteristics was prepared:

Free acid: 7.9 points Total acid: 51.2 points Fe: 1.2 g./l.

Zn++z 11.3 g./l. PO 26.4 g./l. NOy-z 12.5 g./l. Cu++: 0.25 g./l.

Steel panels were soak cleaned in an alkaline cleaner, rinsed in cold water, pickled in hydrochloric acid, again rinsed in water and placed in the coating bath described above for 20 minutes at 190 F.

Followng this pre-coating step, the pre-treated steel specimens were immersed for a period of about 20 minutes at 195 F. in a typical zinc phosphate solution having the following characteristics:

Free acidity: 9.7 points Total acidity: 49.9 points Ferrous iron (Fe++): 3.2 g./l. Zinc (Zn 9.54 g./l.

4 Nitrate (NO 11.2 g./l. Phosphate (POE): 26.3 g./l. Nickel (Ni++): 0.068 g./l.

quality,

Example II In this example a typical production run was made in order to determine the commercial feasibility of the process of the invention. Threaded steel fasteners were loaded into a standard production type processing barrel and then dipped in an alkaline soak cleaner for 3 minutes to remove foreign soils and oils from their surface. After rinsing in water, the fasteners were pickled for three minutes in a hydrochloric acid solution, again rinsed in water and then immersed at a temperature of about F. for 3 minutes in a copper-containing, zinc phosphate coating bath having the following characteristics:

Free acidity: 10.0 points Total acidity: 50.8 points Ferrous iron (Fe++): 1.0 g./l. Zinc (Zn++): 10.0 g./1. Nitrate (NO 9.2 g./l. Phosphate (PO 27.0 g./l. Nickel (Ni 0.075 g./l. Copper (Cu++): 0.282 g./l.

The fasteners were removed from the phosphating bath, rinsed with water, and then immersed in a typical zinc phosphating coating bath for 20 minutes at a temperature of about F. The zinc phosphate bath utilized had the following characteristics:

Free acidity: 9.7 points Total acidity: 49.9 points Ferrous iron (Fe++): 3.2 g./l. Zinc (Zn++): 9.54 g./l. Nitrate (NO 11.2 g./l. Phosphate (PO 26.3 g./l. Nickel (Ni++): 0.068 g./l.

The fasteners were removed from the zinc phosphate bath, again water rinsed and finally immersed in a watersoluble, rust-preventive oil. The coated fasteners exhibited a red-colored zinc phosphate coating of excellent quality easily distinguishable from fasteners of the exact same size processed through a typical zinc phosphate coating bath which results in the production of gray colored fasteners. Fasteners, thus treated, were tested in a neutral salt spray testing cabinet according to the procedure of ASTM B-117 and showed no sign of corrosion after 72 hours exposure.

The chemical make-up of the phosphate solutions employed in the treating baths changed somewhat due to chemical consumption while the fasteners were being processed. The copper content, for example, was in the range of 0.163 to 0.282 gram of copper per liter (introduced as copper sulfate) during the production run.

Example III A second production test was run employing copper in the pre-coating bath at a higher level for the purpose of imparting a more significant red color to the steel fasteners. Otherwise, the phosphate coating compositions were exactly the same as in Example II and the'same processing techniques were used as described in that example.

Fasteners coated in the same manner as in Example II exhibited an intense, uniform, reddish-colored, zinciron phosphate coating.

The copper containing phosphate coating bath employed during this test was held at 0.682 to 0.714 gram of copper per liter, introduced as copper sulfate. During this production run, frequent small additions of copper sulfate were made to maintain the copper at the desired level. In practice, the use of a low volume proportioning pump is recommended to maintain the copper at the required concentration. The processing barrels employed during this particular production run were constructed of stainless steel and a portion of the consumed copper galvanically deposited on the stainless steel. It was also noted that after the processing barrels were removed from the typical coating bath most of the galvanically deposited copper had dissolved and it is believed that it redeposited on the work intended to be coated. The copper retained on the barrels after processing can easily be removed in an alkaline soak cleaner by incorporating a small amount of oxidizing agent, such as sodium meta-nitrobenzene sulfonate and a complexing agent, sodium cyanide. This eliminates the possibility of contaminating the acid pickle with copper which would easily deposit on the work being pickled.

What is claimed is:

1. A process for treating the surface of a ferrous metal, to provide a uniform, reddish-colored, zinc phosphate coating thereon, which comprises:

(a) immersing said metal surface in a first aqueous zinc phosphate bath containing copper in ionic condition to form a phosphate precoat on the said metal surface, the temperature of said first bath being about 120 to about 210 F. and the time of immersion being about 1 minute to about 10 minutes, wherein said first aqueous zinc phosphate bath has the following characteristics:

Free acidity: about 3.5 to about 25 points Total acidity: about 14.5 to about 100 points Fe++z about 0.1 to about 12 g./l.

Zn++: about 1 to about 25 g./l.

P about 5 to about 55 g./l.

N0 about 1 to about 50 g./l.

Cu++: about 0.15 to about 2.5 g./l.

(b) immersing the said precoated metal surface in a second aqueous zinc phosphate bath, the temperature of the said second bath being about 170 to about 210 F. and the time of immersion being about 10 to about minutes, wherein the said second aqueous zinc phosphate bath has the following characteristics:

Free acidity: about 3.5 to about 25 points Total acidity: about 14.5 to about points Fe++: about 0.1 to about 12 g./l.

Zn++z about 1 to about 25 g./l.

P0 about 5 to about 55 g./l.

NO about 1 to about 50 g./l.

2. The process of claim 1 wherein the said ferrous metal is steel.

3. The process of claim 1 wherein the temperature of the bath is about F., the time of immersion is about 20 minutes, and said first phosphate bath has the following characteristics:

Free acid: 7.9 points Total acid: 51.2 points Fe++: 1.2 g./l. Zn++: 11.3 g./l. P0 26.4 g./l. Ne -z 12.5 g./l. Cu++z 0.25 g./l.

4. In the process of claim l wherein in the first said phosphate bath the copper is derived from copper sulfate.

5. The process of claim 1 wherein the said metal is steel.

6. The process of claim 1 wherein in step (a) the temperature of the said bath is about 175 F. and the time of immersion is about 3 minutes and in the step (b) the temperature of the said bath is about F. and the time of immersion is about 20 minutes.

7. A ferrous metal article having the surface produced with the procedure of claim 1.

References Cited UNITED STATES PATENTS 2,272,216 2/ 1942 Lodeesen 1486.l5 Z

2,845,376 7/1958 Hyams 148--6.15 Z

3,467,589 9/1969 Rausch et a1. 1486.15 ZX FOREIGN PATENTS 1,060,693 7/ 1959 Germany 1486.15 Z

RALPH S. KENDALL, Primary Examiner U.S. Cl. X.R. 148-315 

